Semiconductor device and method for fabricating the same

ABSTRACT

A semiconductor device of this invention includes a first interconnect pattern formed on a semiconductor substrate and a second interconnect pattern formed above the first interconnect pattern with an interlayer insulating film sandwiched therebetween. The first interconnect pattern includes a dummy pattern insulated from the first interconnect pattern, and the dummy pattern includes a plurality of fine patterns adjacent to each other and air gaps formed between the adjacent fine patterns.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a semiconductor device having a multi-level interconnect structure, and more particularly, it relates to a semiconductor device including a dummy pattern useful in planarization of an interlayer insulating film formed on an interconnect pattern and a method for fabricating the same.

[0002] Recently, in accordance with improvement in integration and performance of semiconductor devices, interconnect patterns included in a device are refined and formed in multiple levels. Owing to the refinement and the multi-level structure of the interconnect patterns, a level difference caused on the top face of an interlayer insulating film has become large and abrupt, which degrades the processing accuracy and the reliability of the interconnect patterns.

[0003] In particular, a difference in the height (absolute height) between the top face of a wafer and the top face of an interlayer insulating film, namely, the so-called global level difference, is increased due to the multi-level structure of metal interconnects. Furthermore, since exposing light of a shorter wavelength is employed in the lithography as the device is more refined, the depth of focus becomes insufficient. As a result, the processing accuracy and the reliability of the interconnect patterns are degraded. As a planarization technique for an interlayer insulating film for reducing the global level difference, chemical mechanical polishing (CMP) is employed.

[0004] Furthermore, the refinement and the multi-level structure of the interconnect patterns in accordance with the increase in integration level of semiconductor devices can increase capacitance between interconnects. The increase of the capacitance between interconnects affects the operation speed of the semiconductor device, and hence, the capacitance between interconnects needs to be reduced. In order to reduce the capacitance between interconnects, an insulating material with a low dielectric constant is used, and in order to further reduce the capacitance between interconnects, an air gap interconnect structure where air gaps are provided between interconnect patterns are employed.

[0005] The air gap interconnect structure is reported in, for example, papers written by T. Ueda et al. (A Novel Air Gap Integration Scheme for Multi-level Interconnects using Self-aligned Via Plugs: 1988 Symposium on VLSI Technology Digest of Technical Papers, P. 46, 1998; and Integration of 3 Level Air Gap Interconnect for Sub-quarter Micron CMOS: 1999 Symposium on VLSI Technology Digest of Technical Papers, P. 111, 1999).

[0006] Also in the formation of air gap multi-level interconnects, it is necessary to planarize an interlayer insulating film. In the planarization by the CMP, a dummy pattern is disposed in a portion where interconnect patterns are relatively sparse.

[0007] A conventional semiconductor device having the an air gap interconnect structure including a dummy pattern used for the planarization of an interlayer insulating film will now be described with reference to accompanying drawings.

[0008]FIGS. 7A through 7C and 8A through 8C are cross-sectional views for showing procedures in a method for fabricating the conventional semiconductor device including an air gap interconnect pattern and a dummy pattern.

[0009] First, as shown in FIG. 7A, a first insulating film 102 of silicon oxide, a conducting film 103 of aluminum alloy and a second insulating film 104 of silicon oxide are successively deposited on a semiconductor substrate 101 of silicon. Subsequently, a contact hole 104 a for exposing the conducting film 103 is selectively formed in the second insulating film 104.

[0010] Next, as shown in FIG. 7B, the contact hole 104 a is filled with tungsten, thereby forming a plug 105.

[0011] Then, as shown in FIG. 7C, the second insulating film 104 is etched back so as to expose an upper portion of the plug 105. The position of the top of an air gap subsequently formed in the interconnect pattern is determined by controlling the thickness of the second insulating film 104 remaining after the etch back.

[0012] Subsequently, as shown in FIG. 8A, a resist pattern 121A for a first interconnect pattern and a resist pattern 121B for a dummy pattern are formed on the second insulating film 104. With the resist patterns 121A and 121B and the plug 105 used as a mask, the second insulating film 104, the conducting film 103 and the first insulating film 102 are successively etched, thereby forming, from the conducting film 103, a first interconnect pattern 103 a and a dummy pattern 103 b having a square plane shape. At this point, a portion of the first insulating film 102 sandwiched between patterns of the first interconnect pattern 103 a is trenched by the etching. Accordingly, an air gap can be easily formed from a space between the patterns and the position of the air gap from the substrate surface can be adjusted.

[0013] Next, as shown in FIG. 8B, after removing the resist patterns 121A and 121B, a third insulating film 107 of silicon oxide with low coverage and high directivity is deposited on the entire surface of the semiconductor substrate 101. Thereafter, a fourth insulating film 108 of silicon oxide with high coverage is formed. At this point, an air gap 122 is formed between the patterns of the first interconnect pattern 103 a. Thus, the air gap interconnect structure is obtained.

[0014] Then, as shown in FIG. 8C, the top face of the fourth insulating film 108 is polished and planarized by the CMP until the top face of the plug 105 is exposed.

[0015] In the fabrication method for the conventional semiconductor device, however, although the dummy pattern 103 b is formed in a region where the first interconnect pattern 103 a is relatively sparse on the semiconductor substrate 101, a difference h1 in the absolute height of the top face of the fourth insulating film 108 between a portion above the first interconnect pattern 103 a and a portion above the dummy pattern 103 b cannot be sufficiently small as shown in FIG. 8B. Therefore, even though the fourth insulating film 108 is planarized, there remains a difference h2 in the absolute height as shown in FIG. 8C. Thus, a sufficiently planar face cannot be attained.

[0016] When a second interconnect pattern is formed on the fourth insulating film 108 with such a difference h2 remaining, interconnects included in the second interconnect pattern may be disconnected due to the difference h2 or may fail due to high resistance. In order to more sufficiently planarize the top face of the fourth insulating film 108, a larger number of dummy patterns 103 b may be provided. When a larger number of dummy patterns 103 b are provided, however, capacitance between interconnect layers and between interconnects become so large that it is difficult to attain high operation speed. Therefore, the number of dummy patterns 103 b is not preferably increased.

SUMMARY OF THE INVENTION

[0017] The invention was devised to overcome the aforementioned conventional problems, and an object is reducing a difference in the absolute height caused on the top face of an interlayer insulating film after planarization with the increase of capacitance between interconnect layers and between interconnects suppressed.

[0018] The present inventors have variously studied the reason why the difference h1 in the absolute height shown in FIG. 8B cannot be reduced to an allowable value in the conventional semiconductor device including the dummy pattern provided in a region where air gap interconnects are comparatively sparse, resulting in finding out the following cause:

[0019] In the air gap interconnect structure, since the air gap 122 is provided between the patterns of the first interconnect pattern 103 a, the absolute height of a region where the first interconnect pattern 103 a is relatively dense is larger than the absolute height of an interconnect pattern including no air gap. Accordingly, even though the dummy pattern 103 b is formed in the region where the first interconnect pattern 103 a is relatively sparse, the difference h1 in the absolute height from the portion above the dummy pattern is larger than that in the interconnect pattern including no air gap.

[0020] According to the invention, in order to reduce the difference h1 in the absolute height, a dummy pattern is formed as a plurality of fine patterns adjacent to each other with a space therebetween and the space is filled with nothing to form an air gap.

[0021] Specifically, the semiconductor device of this invention comprises a first interconnect pattern formed on a semiconductor substrate; and a second interconnect pattern formed above the first interconnect pattern with an interlayer insulating film sandwiched therebetween, and the first interconnect pattern includes a dummy pattern electrically insulated from the first interconnect pattern, and the dummy pattern includes a plurality of fine patterns adjacent to each other and air gaps sandwiched between the adjacent fine patterns.

[0022] In the semiconductor device of the invention, although the first interconnect pattern has the air gap interconnect structure where air gaps are formed between interconnect patterns, the absolute height of the top face of the interlayer insulating film in a portion above the dummy pattern is larger than that obtained without forming air gaps, and hence, a difference in the absolute height of the interlayer insulating film from a portion above the first interconnect pattern can be sufficiently small. Accordingly, even though the first interconnect pattern has the air gap structure, it can be sufficiently planarized by polishing, so that a good second interconnect pattern can be definitely formed on the sufficiently planarized interlayer insulating film without causing a defect such as disconnection. Moreover, since the dummy pattern itself has the air gap structure, the increase of the capacitance between interconnects and between interconnect layers due to the dummy pattern can be suppressed. Furthermore, when the dummy pattern of this invention is provided so as to attain capacitance between interconnect layers and between interconnects equivalent to those attained by a dummy pattern including no air gaps, a larger number of dummy patterns can be disposed in a region where the first interconnect pattern is comparative sparse. As a result, the interlayer insulating film can attain a further planar face.

[0023] In the semiconductor device, air gaps are preferably provided between adjacent patterns of the first interconnect pattern.

[0024] In the semiconductor device, the dummy pattern is preferably formed in a region on the semiconductor substrate where the first interconnect pattern is relatively sparse.

[0025] The first method for fabricating a semiconductor device of this invention comprises a first step of forming a first interconnect pattern forming layer from a conducting film on a semiconductor substrate and a first insulating film on the first interconnect pattern forming layer; a second step of forming a plug electrically connected to the first interconnect pattern forming layer by forming an opening in the first insulating film by selective etching and filling the opening with a conducting film; a third step of forming a resist pattern corresponding to a first interconnect pattern including a dummy pattern from a resist film applied over the semiconductor substrate including the plug; a fourth step of patterning the first insulating film by etching the first insulating film with the resist pattern and the plug used as a mask; a fifth step of forming, from the first interconnect pattern forming layer, the first interconnect pattern and the dummy pattern that is electrically insulated from the first interconnect pattern and includes a plurality of fine patterns adjacent to each other by etching the first interconnect pattern forming layer with the resist pattern and the plug used as a mask; a sixth step of forming, on the semiconductor substrate, a second insulating film covering the plug, the first interconnect pattern and the dummy pattern so as to form air gaps between patterns of the first interconnect pattern and between the fine patterns of the dummy pattern; and a seventh step of planarizing a top face of the second insulating film until the plug is exposed and forming, on the second insulating film, a second interconnect pattern electrically connected to the plug after planarization.

[0026] In the first method for fabricating a semiconductor device, the first interconnect pattern and the dummy pattern that is electrically insulated from the first interconnect pattern and includes a plurality of fine patterns adjacent to each other are formed from the first interconnect pattern forming layer, and thereafter, the second insulating film is formed so as to form air gaps between patterns of the first interconnect pattern and the fine patterns of the dummy pattern. Accordingly, a difference in the absolute height of the top face of the second insulating film between a portion above the dummy pattern and a portion above the first interconnect pattern can be sufficiently small. As a result, the difference in the absolute height is sufficiently small even after the planarization of the second insulating film, and hence, no failure is caused in the second interconnect pattern formed on the second insulating film sufficiently planarized. Furthermore, since the dummy pattern itself includes the air gaps, the increase of the capacitance between interconnects and between interconnect layers due to the dummy pattern can be suppressed.

[0027] In the method for fabricating a semiconductor device, the first step preferably includes a sub-step of forming an underlying insulating film on the semiconductor substrate before forming the first interconnect pattern forming layer, and the method preferably further includes, between the fifth step and the sixth step, a step of trenching an upper portion of the underlying insulating film by etching the underlying insulating film with the resist pattern and the plug used as a mask. In this manner, each space between the patterns of the first interconnect pattern and between the fine patterns of the dummy pattern can attain a high aspect ratio, and hence, the air gaps can be easily formed between the patterns in the sixth step. Furthermore, the position from the semiconductor substrate (height) of the air gap can be thus controlled.

[0028] The second method for fabricating a semiconductor device of this invention comprises a first step of forming a first interconnect pattern forming layer of a conducting film on a semiconductor substrate; a second step of forming a resist pattern corresponding to a first interconnect pattern including a dummy pattern from a resist film applied over the semiconductor substrate; a third step of forming, from the first interconnect pattern forming layer, the first interconnect pattern and the dummy pattern that is electrically insulated from the first interconnect pattern and includes a plurality of fine patterns adjacent to each other by etching the first interconnect pattern forming layer with the resist pattern used as a mask; a fourth step of forming, on the semiconductor substrate, an insulating film covering the first interconnect pattern and the dummy pattern so as to form air gaps between the fine patterns of the dummy pattern; a fifth step of planarizing a top face of the insulating film and forming a plug electrically connected to the first interconnect pattern by forming an opening in the insulating film after planarization by selective etching and filling the opening with a conducting film; and a sixth step of forming, on the insulating film, a second interconnect pattern electrically connected to the plug.

[0029] In the second method for fabricating a semiconductor device, the first interconnect pattern and the dummy pattern that is electrically insulated from the first interconnect pattern and includes a plurality of fine patterns adjacent to each other are formed from the first interconnect pattern forming layer, and thereafter, the insulating film is formed so as to form air gaps between the fine patterns of the dummy pattern. Therefore, a difference in the absolute height of the top face of the insulating film between a portion above the dummy pattern and a portion above the first interconnect pattern can be sufficiently small. As a result, the difference in the absolute height can be sufficiently small even after the planarization of the insulating film, and hence, no failure is caused in the second interconnect pattern formed on the insulating film sufficiently planarized. In addition, since the dummy pattern itself includes the air gaps, the increase of the capacitance between interconnects and between interconnect layers due to the dummy pattern can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIGS. 1A and 1B are diagrams of a mask used in fabrication of a semiconductor device according to Embodiment 1 of the invention, wherein FIG. 1A is a plane view of an interconnect pattern and a dummy pattern and FIG. 1B is an enlarged plane view of the dummy pattern;

[0031]FIGS. 2A, 2B, 2C and 2D are cross-sectional views for showing procedures in a method for fabricating the semiconductor device of Embodiment 1;

[0032]FIGS. 3A, 3B and 3C are cross-sectional views for showing other procedures in the method for fabricating the semiconductor device of Embodiment 1;

[0033]FIGS. 4A, 4B and 4C are cross-sectional views for showing procedures in a method for fabricating a semiconductor device according to Embodiment 2 of the invention;

[0034]FIGS. 5A, 5B and 5C are cross-sectional views for showing other procedures in the method for fabricating a semiconductor device of Embodiment 2;

[0035]FIGS. 6A, 6B and 6C are cross-sectional views for showing still other procedures in the method for fabricating a semiconductor device of Embodiment 2;

[0036]FIGS. 7A, 7B and 7C are cross-sectional views for showing procedures in a method for fabricating a conventional semiconductor device including an air gap interconnect pattern and a dummy pattern; and

[0037]FIGS. 8A, 8B and 8C are cross-sectional views for showing other procedures in the method for fabricating the conventional semiconductor device including an air gap interconnect pattern and a dummy pattern.

DETAILED DESCRIPTION OF THE INVENTION

[0038] Embodiment 1

[0039] Embodiment 1 of the invention will now be described with reference to the accompanying drawings.

[0040]FIGS. 1A and 1B show a mask used in fabrication of a semiconductor device according to Embodiment 1 of the invention, wherein FIG. 1A is a plane view of an interconnect pattern and a dummy pattern and FIG. 1B is an enlarged plane view of the dummy pattern.

[0041] As shown in FIG. 1A, the interconnect pattern 10 included in the mask 1 consists of an interconnect part 20 and dummy patterns 30 disposed in a region where patterns of the interconnect part 20 are comparatively sparse. The respective dummy patterns 30 are arranged, for example, at spacing of 8.0 μm and a pitch of 10.0 μm.

[0042] As shown in FIG. 1B, the dummy pattern 30 includes three stripe-shaped fine patterns 30 a each with a length of approximately 2.0 μm and a width of approximately 0.4 μm, and the three fine patterns 30 a arranged at spacing 30 b of approximately 0.4 μm between their longer sides are disposed so as to have a plane outline in a square shape of approximately 2.0 μm×2.0 μm. The plane outline of each dummy pattern 30 is a square in this embodiment, which does not limit the invention. Although each dummy pattern 30 includes the three fine patterns 30 a, the number of fine patterns 30 a is not limited to three but may be two or more.

[0043] A method for fabricating a semiconductor device by using the mask 1 having the aforementioned interconnect pattern 10 will now be described with reference to the accompanying drawings.

[0044]FIGS. 2A through 2D and 3A through 3C are cross-sectional views for showing procedures in the method for fabricating a semiconductor device having an air gap interconnect structure and including a dummy pattern according to this embodiment.

[0045] First, as shown in FIG. 2A, an underlying insulating film 52 of silicon oxide is deposited all over an interconnect region on a semiconductor substrate 51 of silicon by, for example, plasma CVD. Thereafter, a first interconnect pattern forming layer 53 of aluminum alloy is deposited all over the underlying insulating film 52 by, for example, sputtering. Subsequently, a first insulating film 54 of, for example, silicon oxide is deposited on the first interconnect pattern forming layer 53. Then, a resist film (not shown) is applied on the first insulating film 54, thereby forming a mask pattern for a plug by photolithography. The first insulating film 54 is etched by using the thus formed mask pattern, thereby forming a contact hole 54 a in the first insulating film 54.

[0046] Next, as shown in FIG. 2B, a conducting film of, for example, tungsten is deposited all over the first insulating film 54 including the contact hole 54 a by vapor deposition or the like, so as to fill the contact hole 54 a with the deposited conducting film. Subsequently, an excessive portion of the conducting film deposited on the first insulating film 54 is polished and removed by CMP, thereby forming a plug 55 of tungsten in the contact hole 54 a.

[0047] Then, as shown in FIG. 2C, the first insulating film 54 is etched back so that at least an upper portion of the plug 55 can be exposed. The position of the top of an air gap subsequently formed in an interconnect pattern can be determined by controlling the thickness of the first insulating film 54 remaining after the etch back.

[0048] Next, as shown in FIG. 2D, a resist pattern 71A for a first interconnect pattern and a resist pattern 71B for a dummy pattern are formed on the first insulating film 54. By using the resist pattern 71A for a first interconnect pattern and the resist pattern 71B for a dummy pattern as a mask, the first insulating film 54, the first interconnect pattern forming layer 53 and the underlying insulating film 52 are successively etched. Thus, a first interconnect pattern 53 a including spaces 53 c between adjacent patterns and a dummy pattern 53 b electrically insulated from the first interconnect pattern 53 a and including three fine patterns each having a stripe plane shape and spaces 53 d between adjacent fine patterns are formed from the first interconnect pattern forming layer 53. At this point, portions of the underlying insulating film 52 exposed in the spaces 53 c and 53 d of the first interconnect pattern 53 a and the dummy pattern 53 b are trenched by the etching. Therefore, the space 53 c of the first interconnect pattern 53 a also attains a high aspect ratio and the space 53 d of the dummy pattern 53 b originally has a high aspect ratio. As a result, air gaps can be easily formed in the spaces 53 c in a subsequent procedure and the positions of the air gaps from the substrate surface can be optimized.

[0049] Furthermore, since a portion of the underlying insulating film 52 below the side faces of the first interconnect pattern 53 a is removed, a lower fringe component of capacitance between adjacent interconnects can be lowered.

[0050] Subsequently, as shown in FIG. 3A, after removing the resist patterns 71A and 71B, a second insulating film 57 of silicon oxide for forming air gaps is deposited all over the semiconductor substrate 51 including the plug 55, the first interconnect pattern 53 a and the dummy pattern 53 b by plasma CVD using a monosilane (SiH₄) gas and a dinitrogen monoxide (N₂O) gas. At this point, the second insulating film 57 is preferably formed from a film with low coverage and high directivity. When such an insulating film with low coverage is used, the deposited second insulating film 57 overhangs like a peak on the top portions of the adjacent patterns, and hence, an air gap can be definitely formed between the patterns. Thereafter, a third insulating film 58, which may have high coverage, of silicon oxide to be used as an interlayer insulating film is deposited all over the second insulating film 57 by, for example, high density plasma CVD. As a result, air gaps 72 are formed between the patterns of the first interconnect pattern 53 a so as to attain the air gap interconnect structure, and air gaps 73 are also formed between the fine patterns of the dummy pattern 53 b so that the dummy pattern 53 b can also attain the air gap structure.

[0051] Also in this embodiment, there is a difference H1 in the absolute height of the top face of the third insulating film 58 between a portion above the first interconnect pattern 53 a and a portion above the dummy pattern 53 b. However, since the dummy pattern 53 b includes a plurality of fine patterns and the spaces 53 d are formed between the adjacent fine patterns, a portion of the third insulating film 58 not filled in the spaces 53 d rises in the portion above the dummy pattern 53 b, and hence, the absolute height of the portion above the dummy pattern 53 b is larger than that obtained without employing the air gap structure. The difference H1 is smaller than the difference h1 obtained by the conventional dummy pattern 103 b shown in FIG. 8B, and hence, the deposited third insulating film 58 is superior in the planeness on its top face.

[0052] Next, as shown in FIG. 3B, the top face of the third insulating film 58 is polished and planarized by the CMP until the top face of the plug 55 is exposed. At this point, since the difference H1 in the absolute height is sufficiently allowably small, a difference H2 in the absolute height attained after the planarization is very small. Thus, a sufficiently planar face can be obtained.

[0053] Then, as shown in FIG. 3C, a conducting film electrically connected to the plug 55 is formed on the planarized third insulating film 58 by the sputtering or the like, and a second interconnect pattern 59 is formed from the conducting film. At this point, since the third insulating film 58 is sufficiently planarized, the second interconnect pattern 59 is never disconnected owing to the level difference of the third insulating film 58. Accordingly, the second interconnect pattern 58, that is, an upper layer interconnect pattern, can be definitely formed.

[0054] In this manner, according to the fabrication method of Embodiment 1, the dummy pattern 53 b including the air gaps 73 between plural fine patterns can be definitely formed in a pattern region where the first interconnect pattern 53 a is comparatively sparse in the semiconductor device.

[0055] Also, since the second insulating film 57 with high directivity is used for forming the air gaps and the third insulating film 58 with low directivity is used for forming the interlayer insulating film in this embodiment, the controllability in the positions of the tops of the air gaps 72 and 73 can be improved. In addition, even when the interconnects are formed at comparatively large spacing, the air gaps 72 and 73 can be definitely formed. The interlayer insulating film may be formed from a single-layer insulating film, and in this case, a silicon oxide film with relatively low coverage is preferably used as the interlayer insulating film.

[0056] Although the two-level interconnect structure including the first interconnect pattern 53 a and the second interconnect pattern 59 is formed in this embodiment, the number of the interconnect layers is not limited to two. Any multi-level air gap interconnect structure including three or more layers can be formed by forming a dummy pattern similar to the dummy pattern 53 b in a pattern region where the second interconnect pattern 59 is comparatively sparse.

[0057] In order to increase the size of the air gap 73 of the dummy pattern 53 b, the spacing 30 b of the dummy pattern 30 shown in FIG. 1B is preferably set to 0.75 μm or less.

[0058] Embodiment 2

[0059] A method for fabricating a semiconductor device according to Embodiment 2 of the invention will now be described with reference to the accompanying drawings.

[0060]FIGS. 4A through 4C, 5A through 5C and 6A through 6C are cross-sectional views for showing procedures in the method for fabricating a semiconductor device including an air gap dummy pattern according to Embodiment 2.

[0061] In Embodiment 2, an interconnect pattern does not include air gaps, and a plug for electrically connecting a lower layer interconnect pattern to an upper layer interconnect pattern is formed after forming the lower layer interconnect pattern and a dummy pattern.

[0062] First, as shown in FIG. 4A, an underlying insulating film 62 of silicon oxide is deposited all over an interconnect region on a semiconductor substrate 61 of silicon by, for example, the plasma CVD. Thereafter, a first interconnect pattern forming layer 63 of aluminum alloy is deposited all over the underlying insulating film 62 by, for example, the sputtering.

[0063] Next, as shown in FIG. 4B, a resist pattern 71A for a first interconnect pattern and a resist pattern 71B for a dummy pattern are formed on the first interconnect pattern forming layer 63 by the photolithography. By using the resist pattern 71A for a first interconnect pattern and the resist pattern 71B for a dummy pattern as a mask, the first interconnect pattern forming layer 63 is etched. Thus, a first interconnect pattern 63 a and a dummy pattern 63 b electrically insulated from the first interconnect pattern 63 a and including three fine patterns each having a stripe plane shape and spaces 63 d between adjacent fine patterns are formed from the first interconnect pattern forming layer 63.

[0064] Then, as shown in FIG. 4C, after removing the resist patterns 71A and 71B, a first insulating film 64 of silicon oxide for forming air gaps is deposited all over the semiconductor substrate 61 including the first interconnect pattern 63 a and the dummy pattern 63 b by the plasma CVD using a monosilane (SiH₄) gas and a dinitrogen monoxide (N₂O) gas. At this point, the first insulating film 64 is preferably formed from a film with low coverage and high directivity. When such an insulating film with low coverage is used, the deposited first insulating film 64 overhangs like a peak on the top portions of the adjacent fine patterns of the dummy pattern 63 b, and hence, air gaps can be definitely formed between the fine patterns.

[0065] Thereafter, as shown in FIG. 5A, a second insulating film 65, which may be an insulating film with comparatively high coverage, of silicon oxide to be used as an interlayer insulating film is deposited all over the semiconductor substrate 61 including the first insulating film 64 by the high density plasma CVD. Thus, air gaps 73 are formed between the fine patterns of the dummy pattern 63 b, and the dummy pattern 63 b attains the air gap structure.

[0066] Since the dummy pattern 63 b thus includes a plurality of fine patterns and the air gaps 73 provided between the adjacent fine patterns, the second insulating film 65 is not filled in the air gaps 73 of the dummy pattern 63 b. Therefore, a difference H1 in the absolute height becomes smaller than in the conventional dummy pattern including no air gaps. As a result, the deposited second insulating film 65 attains good planeness on its top face.

[0067] Next, as shown in FIG. 5B, the top face of the second insulating film 65 is polished and planarized by the CMP. At this point, since the difference H1 in the absolute height attained before the polishing is sufficiently small, a difference H2 in the absolute height attained after polishing and planarizing the top face of the second insulating film 65 is very small. Thus, a sufficiently planar face can be obtained.

[0068] Then, as shown in FIG. 5C, a resist film is applied on the second insulating film 65, and a mask pattern 75 for a plug having an opening 75 a above the first interconnect pattern 63 a is formed by the photolithography.

[0069] Subsequently, as shown in FIG. 6A, the second insulating film 65 and the first insulating film 64 are etched by using the mask pattern 75 for a plug, thereby forming a contact hole 65 a in the second insulating film 65 above an interconnect of the first interconnect pattern 63 a.

[0070] Next, as shown in FIG. 6B, a conducting film of, for example, tungsten is deposited all over the second insulating film 65 including the contact hole 65 a by the vapor evaporation or the like, so as to fill the deposited conducting film in the contact hole 65 a . Subsequently, an excessive portion of the conducting film deposited on the second insulating film 65 is polished and removed by the CMP, thereby forming a plug 66 of tungsten in the contact hole 65 a.

[0071] Then, as shown in FIG. 6C, a conducting film electrically connected to the plug 66 is formed on the planarized second insulating film 65 by the sputtering or the like, and a second interconnect pattern 67 is formed from the conducting film. At this point, since the second insulating film 65 is sufficiently planarized, the second interconnect pattern 67 is never disconnected owing to a level difference in the second insulating film 65. Thus, the second interconnect pattern 67, that is, an upper interconnect pattern, can be definitely formed.

[0072] In this manner, according to the fabrication method of Embodiment 2, the dummy pattern 63 b including the air gaps 73 between a plurality of fine patterns can be definitely formed in a pattern region where the first interconnect pattern 63 a is comparatively sparse in the semiconductor device.

[0073] Also, since the first insulating film 64 with high directivity is used for forming the air gaps and the second insulating film 65 with low directivity is used for forming the interlayer insulating film in this embodiment, the controllability in the positions of the tops of the air gaps 73 can be improved. In addition, even when the spacing between the fine patterns of the dummy pattern 63 b is comparatively large, the air gaps 73 can be definitely formed. The interlayer insulating film may be formed from a single-layer film, and in this case, a silicon oxide film with relatively low coverage is preferably used for the interlayer insulating film.

[0074] Although the two-level interconnect structure including the first interconnect pattern 63 a and the second interconnect pattern 67 is formed in this embodiment, the number of layers of the interconnects is not limited to two. Any multi-level air gap interconnect structure including three or more layers can be formed by forming a dummy pattern similar to the dummy pattern 63 b in a pattern region where the second interconnect pattern 67 is comparatively sparse.

[0075] In order to increase the size of the air gap 73 of the dummy pattern 63 b, the spacing 30 b of the dummy pattern 30 shown in FIG. 1B is preferably set to 0.75 μm or less. 

What is claimed is:
 1. A method far fabricating a semiconductor device comprising: a first step of forming a first interconnect pattern forming layer from a conducting film on a semiconductor substrate and a first insulating film on said first interconnect pattern forming layer; a second step of forming a plug electrically connected to said first interconnect pattern forming layer by forming an opening in said first insulating film by selective etching and filling said opening with a conducting film; a third step of forming a resist pattern corresponding to a first interconnect pattern including a dummy pattern from a resist film applied over said semiconductor substrate including said plug; a fourth step of patterning said first insulating film by etching said first insulating film with said resist pattern and said plug used as a mask; a fifth step of forming, from said first interconnect pattern forming layer, said first interconnect pattern and said dummy pattern that is electrically insulated from said first interconnect pattern and includes a plurality of fine patterns adjacent to each other by etching said first interconnect pattern forming layer with said resist pattern and said plug used as a mask; a sixth step of forming, on said semiconductor substrate, a second insulating film covering said plug, said first interconnect pattern and said dummy pattern so as to form air gaps between patterns of said first interconnect pattern and between said fine patterns of said dummy pattern; and a seventh step of planarizing a top face of said second insulating film until said plug is exposed and forming, on said second insulating film, a second interconnect pattern electrically connected to said plug after planarization.
 2. The method for fabricating a semiconductor device of claim 1, wherein said first step includes a sub-step of forming an underlying insulating film on said semiconductor substrate before forming said first interconnect pattern forming layer, and the method further includes, between said fifth step and said sixth step, a step of trenching an upper portion of said underlying insulating film by etching said underlying insulating film with said resist pattern and said plug used as a mask.
 3. A method for fabricating a semiconductor device comprising: a first step of forming a first interconnect pattern forming layer of a conducting film on a semiconductor substrate; a second step of forming a resist pattern corresponding to a. first interconnect pattern including a dummy pattern from a resist film applied over said semiconductor substrate; a third step of forming, from said first interconnect pattern forming layer, said first interconnect pattern and said dummy pattern that is electrically insulated from said first interconnect pattern and includes a plurality of fine patterns adjacent to each other by etching said first interconnect pattern forming layer with said resist pattern used as a mask; a fourth step of forming, on said semiconductor substrate, an insulating film covering said first interconnect pattern and said dummy pattern so as to form air gaps between said fine patterns of said dummy pattern; a fifth step of planarizing a top face of said insulating film and forming a plug electrically connected to said first interconnect pattern by forming an opening in said insulating film after planarization by selective etching and filling said opening with a conducting film; and a sixth step of forming, on said insulating film, a second interconnect pattern electrically connected to said plug.
 4. A method for fabricating a semiconductor device comprising: a first step of forming a first interconnect pattern forming layer on a substrate, and a first insulating film on said first interconnect pattern forming layer; a second step of forming an opening in said first insulating film, and forming a plug electrically connected to said first interconnect pattern forming layer; after said second step, a third step of forming a first interconnect pattern and a dummy pattern from a laminated film composed of said first interconnect pattern forming layer and said first insulating film; after said third step, a fourth step of forming a second insulating film such that an air gap is provided between each said first interconnect pattern and between each said dummy pattern; and a fifth step of planarizing a top face of said second insulating film until said plug is exposed and forming, on said second insulating film, a second interconnect pattern electrically connected to said plug, wherein said dummy pattern is composed of a plurality of fine patterns adjacent to each other, and is electrically insulated from said first interconnect pattern.
 5. The method for fabricating a semiconductor device of claim 4, wherein said first step further includes a step of forming an underlying insulating film below said first interconnect pattern forming layer, and wherein in said third step, a first interconnect pattern and a dummy pattern are formed from said first insulating film, said first interconnect pattern forming layer and an upper portion of said underlying insulating film.
 6. A method for fabricating a semiconductor device comprising: a first step of forming a first interconnect pattern forming layer on a substrate; after said first step, a second step of forming a first interconnect pattern and a dummy pattern from said first interconnect pattern farming layer; after said second step, a third step of forming a second insulating film such that an air gap is provided between each said first interconnect pattern and between each said dummy pattern; a fourth step of planarizing a top face at said second insulating film and farming an opening in said second insulating film, thereafter forming a plug electrically connected to said first interconnect pattern in said opening; and a fifth step of forming a second interconnect pattern electrically connected to said plug, wherein said dummy pattern is composed of a plurality of fine patterns adjacent to each other, and is electrically insulated from said first interconnect pattern. 